This present invention involves use of a high gas permeability polybenzoxazole membrane operated at high temperature for natural gas upgrading (e.g, CO2 removal from natural gas). This membrane can be used in either a single stage membrane or as the first stage membrane in a two stage membrane system for natural gas upgrading. This invention allows the membrane to be operated without a costly pretreatment system.
In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications have achieved commercial success, including carbon dioxide removal from natural gas and from biogas and enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams.
However, early field practice found that membrane performance can deteriorate quickly. The primary cause of loss of membrane performance is liquid condensation on the membrane surface. Condensation is prevented by providing a sufficient dew point margin for operation, based on the calculated dew point of the membrane product gas. UOP's MemGuard™ system, a pretreatment regenerable adsorbent system that uses molecular sieves, was developed to remove water as well as heavy hydrocarbons ranging from C10 to C35 from the natural gas stream, hence, to lower the dew point of the stream. The selective removal of heavy hydrocarbons by a pretreatment system can significantly improve the performance of the membranes.
Although these pretreatment systems can effectively remove heavy hydrocarbons from natural gas streams to lower their dew point, the cost is quite significant. Some projects showed that the cost of the pretreatment system was as high as 10 to 40% of the total cost (pretreatment system and membrane system) depending on the feed composition. Reduction of the pretreatment system cost or total elimination of the pretreatment system would significantly reduce the membrane system cost for natural gas upgrading. On the other hand, in recent years, more and more membrane systems have been applied to large offshore natural gas upgrading projects. For offshore projects, the footprint is a big constraint. Hence, reduction of footprint is very important for offshore projects. The footprint of the pretreatment system is also very high at more than 10-50% of the footprint of the whole membrane system. Removal of the pretreatment system from the membrane system has great economical impact especially to offshore projects.
The membranes most commonly used in commercial gas separation applications are polymeric and nonporous. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. Polymers provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for gas separation. A polymer material with a high glass-transition temperature (Tg), high melting point, and high crystallinity is preferred.
Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability. One issue of the CA membranes is the plasticization of CA polymer with high CO2 concentration in the feed gas that leads to swelling and to an increase in the permeability and a decrease in the selectivity of CA membrane. High-performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole were developed to combine high selectivity and high permeability together with high thermal stability. These polymeric membrane materials have shown promising properties for separation of gas pairs such as CO2/CH4, O2/N2, H2/CH4, and propylene/propane (C3H6/C3H8). However, commercially available polymer membranes can not be operated at elevated temperature because of low mechanical and thermal stability, low selectivity at high temperature.
A recent publication in Science reported on a new type of high permeability polybenzoxazole polymer membranes for gas separations (Ho Bum Park et al, Science 318, 254 (2007)). The polybenzoxazole polymers are prepared from high temperature heat treatment of hydroxyl-containing polyimide polymers containing pendent hydroxyl groups ortho to the heterocyclic imide nitrogen. These polybenzoxazole polymer membranes exhibited extremely high CO2 permeability (>1000 Barrer) for CO2/CH4 separation. This material has very good mechanical and thermal stability at elevated temperature.
The present invention involves a process of treating natural gas using high gas permeability polybenzoxazole polymer membranes operated at high temperatures so that enough dew point margin will be provided for the product gas. Such membrane system can be operated without a pretreatment system, which can significantly save costs and reduce the footprint for the membrane system. This membrane can be used for a single stage membrane system or for the first stage membrane in a two stage membrane system for natural gas upgrading.